Because optical characteristics, such as refraction, diffraction, reflection, polarization and interference, can be described in physics, optical technologies are widely used in various industrial fields. Specifically, a short-wavelength beam can be useful in an industrial field that requires high resolution, such as an exposure process for a semiconductor device. The rapid growth of the semiconductor industry (especially, the manufacture of highly integrated semiconductor devices) in the past decades is a typical example where optical technologies have been applied successfully.
In order to obtain higher resolution, however, a technology that can control light characteristics more precisely may be required. For example, in the case of an exposure process for a semiconductor device, optical characteristics, such as exposure beam intensity, focal length and telecentricity, should be controlled precisely. However, such optical characteristics are influenced by inherent errors in optical/mechanical components constituting the optical system. Because the optical/mechanical components of the optical system may be deformed due to temperature change, gravity, acceleration or abrasion from movement, the enhancement of precision is restricted even though the understanding of beam characteristics is sufficient. Accordingly, a new technology that can enhance the precision is desirable.
In order to overcome the optical precision restrictions, various technologies have been proposed. An exposure system that requires high resolution may demand a strict control of telecentricity. Telecentricity is an optical characteristic associated with change in image magnification or shape distortion. FIGS. 1A and 1B are views for explaining an optical system having a perfect telecentricity. FIGS. 2A, 2B, 3A and 3B are views for explaining optical systems having two types of non-telecentricity.
Referring to FIGS. 1A and 1B, beams 10a are vertically incident on a reference plane 5 in parallel to one another. In this case, images formed by the beams 10a have identical shape and size without regard to positions of the reference plane (that is, Wa1=Wa2, Wax1=Way1=Wax2=Way2). By contrast, as illustrated in FIGS. 2A and 2B, when beams 10b are not in parallel, the beams 10b are incident on a reference plane 5 at various angles. In this case, when the position of the reference plane 5 is changed, an image may be increased or decreased in size (that is, Wb1≠Wb2, Wbx1=Wby1, Wbx2=Wby2, Wby1≠Wby2). In the case of such a non-telecentricity, the magnification may be changed depending on the change in focal length. However, because recent exposure systems can precisely control the focal length, a type of non-telecentricity as illustrated in FIGS. 3A and 3B is experienced more often than that illustrated by FIGS. 2A and 2B.
Referring to FIG. 3A, although beams 10c are in parallel to one another, the beams 10c are deviated from a predetermined reference direction and are incident on a reference plane 5 at an angle thereto. In this case, as illustrated in FIG. 3B, an image may change from a circular shape to an elliptical shape (that is, Wc1=Wc2, Wc1≠Wcp, Wcx1=Wcy1, Wcx2≠Wcy2).
In addition, resolution of a lithography process can be expressed as Equation 1 below
                    resolution        =                              k            1                    ⁢                      λ                          N              ⁢                                                          ⁢              A                                                          (                  Equation          ⁢                                          ⁢          1                )            
From Equation 1, the resolution of the lithography process is proportional to a process constant k1 and a wavelength λ, and is inversely proportional to a numerical aperture (NA). Accordingly, reduction of the process constant is required to enhance the resolution. However, because recent exposure systems perform an exposure process using even an edge of a lens so as to reduce the process constant k1, exposure beams used in the exposure process may be deviated from the lens when the exposure systems have non-telecentricity as illustrated in FIG. 3A.
Accordingly, there is a demand for a technology that can precisely measure the non-telecentricity (aberration of beam) illustrated in FIG. 3A. The measurement of the non-telecentricity can be achieved by measuring a deformation of an illumination formed on a defocused image plane, or by transferring an alignment key or overlay key of a photo mask on a wafer and measuring the transferred image.
With reference to FIG. 4, in a method for measuring deformation of illumination, the degree of the deformation can be difficult to quantify due to the influence from illumination components 22, such as a fly-eye lens, a cylinder lens, a mirror and a condensing lens, which are arranged between a light source 21 and a reticle 23. For example, since a boundary of an image formed on an image plane 25 becomes obscured due to the influence of the illumination components 22, it may be difficult to objectively quantify the degree of the deformation.
FIG. 5 is a flowchart illustrating a method of measuring the transferred image of an alignment key or overlay key. Referring to FIG. 5, in Step 31, a reference pattern is formed using a reference key under a best focus condition. In Step 32, a comparative pattern is formed using a comparative key under a defocus condition. In Step 33, an aberration of an exposure beam is determined by measuring a gap between the reference pattern and the comparative pattern.
At this point, in order to measure the gap, the reference pattern and the comparative pattern must be able to be discriminated with respect to one another. Also, in order to discriminate, the reference key and the comparative key should have different shapes and must be spaced apart from each other a measurable distance. In order to meet those conditions, the wafer is moved such that the comparative pattern is formed in a vicinity of the reference pattern. However, the optical precision provided by an exposure system having high resolution is within an error level of the precision in the movement of the wafer. Therefore, a satisfactory measurement of the resulting aberration cannot be obtained using the method including the step of moving the wafer.